Surface treatment method and casting roll manufactured by the same

ABSTRACT

A surface treatment method capable of preventing a surface defect by improving a surface shape determining surface roughness of a casting roll and a casting roll manufactured by the surface treatment method, are provided. A surface treatment method includes a concave-convex pattern forming operation of forming a concave-convex pattern on a surface of a casting roll; and an electrolytic polishing operation of electrolytically polishing so that a plurality of convex portions and concave portions of a concave-convex pattern formed in the concave-convex pattern forming operation are treated to be rounded.

TECHNICAL FIELD

The present disclosure relates to a surface treatment method of acasting roll used in a twin roll thin plate casting process and acasting roll manufactured by the same.

BACKGROUND ART

In general, a casting process is a process in which molten metal issolidified to manufacture a slab having a uniform thickness. Recently, athin plate casting technique, in which molten metal is directlysolidified as a thin plate having a thickness close to that of a finalproduct by using two casting rolls, has been developed and used.

As illustrated in FIGS. 1 and 2, a twin roll thin plate caster 1supplies molten steel M through a nozzle 20 between a pair of castingrolls 10 rotating in opposite directions, the molten steel M beingcooled along a casting roll 10 to form a casting Y, with reductionrolling being subsequently performed in a roll nip to form a thin plateS.

An edge dam 25 is provided on both ends of the pair of casting rolls 10,to prevent the molten steel M from flowing out toward both ends of thecasting roll 10.

In general, in the casting roll 10, a cooling passage in which a coolantcirculates is formed therein to cool the molten steel M, and the coolingpassage is formed of a material such as copper or the like, havingexcellent heat transfer characteristics.

In addition, a coating layer formed using a material such as nickel orthe like to improve wear resistance properties is formed on a surface ofthe casting roll 10. In the coating layer, non-oriented roughness isformed using a method such as shot blasting, to smoothly dischargebubbles or the like occurring a casting process, and to improvesolidification characteristics by preventing crack defects or the like.

With reference to FIGS. 3 to 5, the casting Y passing through thecasting roll 10 passes through a rolling roll 30 at a temperature of1,100° C. to 1,200° C. to be hot-rolled, and reduction rolling isperformed at a reduction rate of 20% to 40% to manufacture the thinplate S.

In the casting roll 10, surface roughness may be good due to rolling ina process in which the thin plate S is manufactured, and physicalproperties may be good as recrystallization occurs.

For example, the casting roll 10 includes a casting roll main body 12,and a surface treated part having a concave-convex pattern 14 in whichroughness is 10 μm to 20 μm is formed on a surface of the casting rollmain body 12. By the surface treated part of the casting roll 10, aconcave-convex pattern 14′ having surface roughness correspondingthereto, for example, having roughness of 10 μm to 20 μm, is formed onthe casting Y. Roughness of the concave-convex pattern 14′ formed on thecasting Y is lowered to a level of about 1 μm, as the concave-convexpattern 14′ is reduction rolled while passing through the rolling roll30.

However, in the casting Y according to the related art, a convex portionof the concave-convex pattern 14′ may be connected to a concave portionadjacent thereto at an angle (a) of 30 degrees or more. Thus, in aprocess in which the casting Y passes through the rolling roll 30 to berolled to form the thin plate S, a foreign substance such as oxide scaleor the like may be present between convex portions of the concave-convexpattern 14′, and thus, the thin plate S may be manufactured in a statein which a foreign substance is contained therein.

With reference to FIGS. 6A to 6D, as described above, a defect caused bya foreign substance P formed on a surface of the thin plate S isreferred to as a hook defect H. The hook defect H may remain in asubsequent annealing pickling-cold rolling process, and may remain as asurface defect after passing through a final cold rolling process. Thus,such a hook defect H may lead to a value of a product being reduced.

In the casting roll 10 according to the related art, a foreign substancemay be easily inserted into a surface treated part according to avariation, a size, a shape or the like of the concave-convex pattern 14determining surface roughness of the surface treated part. Thus,development of a technique for improving a surface shape so as not toallow a foreign substance to be inserted into a surface treated part ofthe casting roll 10 is required.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a surface treatmentmethod capable of preventing a surface defect by improving a surfaceshape of a casting roll determining surface roughness, and a castingroll manufactured by the method.

Technical Solution

According to an aspect of the present disclosure, a surface treatmentmethod may include: a concave-convex pattern forming operation offorming a concave-convex pattern on a surface of a casting roll; and anelectrolytic polishing operation of electrolytically polishing so that aplurality of convex portions and concave portions of a concave-convexpattern formed in the concave-convex pattern forming operation aretreated to be rounded.

In the concave-convex pattern forming operation, a surface of thecasting roll may be surface-treated using a shot blasting process.

In the shot blasting process of the concave-convex pattern formingoperation, when the casting roll is surface-treated, a shot ball havinga diameter of at least 2.8 mm may be projected.

According to another aspect of the present disclosure, a casting rollmay include: a casting roll main body; and a surface treated part formedon a surface of the casting roll main body and formed so that aplurality of convex portions and concave portions of a concave-convexpattern are treated to be rounded by the surface treatment methoddescribed above.

Convex portions of the concave-convex pattern may be smoothly connectedto concave portions of the concave-convex pattern adjacent thereto at anangle of less than 30 degrees.

The casting roll may further include a coating layer formed on thesurface of the casting roll main body.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, a shapeof a convex portion determining surface roughness of a surface treatedpart may be improved to prevent a phenomenon of insertion of a foreignsubstance. Thus, a hook defect H occurring on a surface of a cold-rolledproduct or the like may be reduced, thereby improving product quality.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a twin roll thin plate caster.

FIG. 2 is a perspective view illustrating a casting roll according tothe related art.

FIG. 3 is a cross-sectional view illustrating a process in which acasting cast by a casting roll according to the related art is pressedas a thin plate.

FIG. 4 is a cross-sectional view taken along line I-I′ of FIG. 3.

FIG. 5 is a cross-sectional view taken along line II-II′ of FIG. 3.

FIG. 6A is a drawing illustrating a hook defect formed on a surface of athin plate, FIG. 6B is a drawing illustrating a cross section of a hookdefect, FIG. 6C is a drawing illustrating a concave-convex pattern of athin plate surface in which a hook defect occurs, and FIG. 6D is a graphillustrated by a height/a width of a concave-convex pattern of a castingroll.

FIG. 7 is a perspective view illustrating a casting roll according to anexemplary embodiment.

FIG. 8 is a cross-sectional view illustrating a method of manufacturinga casting roll according to an exemplary embodiment.

FIG. 9 is a cross-sectional view illustrating a process in which acasting cast by a casting roll according to an exemplary embodiment ispressed as a thin plate.

FIG. 10 is a cross-sectional view taken along line III-III′ of FIG. 9.

FIG. 11 is a cross-sectional view taken along line IV-IV′ of FIG. 9.

BEST MODE FOR INVENTION

Hereinafter, an exemplary embodiment of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Anexemplary embodiment of the present disclosure may be modified to havevarious other forms, and the scope of the present disclosure is notlimited to an exemplary embodiment described hereinafter. A shape, asize or the like of the elements in drawings may be exaggerated forclarity of description, and elements represented by the same referencenumerals in the drawings are the same.

FIG. 7 is a perspective view illustrating a casting roll according to anexemplary embodiment, FIG. 8 is a cross-sectional view illustrating amethod of manufacturing a casting roll according to an exemplaryembodiment, and FIG. 9 is a cross-sectional view illustrating a processin which a casting cast by a casting roll according to an exemplaryembodiment is pressed as a thin plate. In addition, FIG. 10 is across-sectional view taken along line III-III′ of FIG. 9, and FIG. 11 isa cross-sectional view taken along line IV-IV′ of FIG. 9.

With reference to FIGS. 7 to 11, a surface treatment method according toan exemplary embodiment may include a concave-convex pattern formingoperation and an electrolytic polishing operation.

In an exemplary embodiment, in the concave-convex pattern formingoperation, a concave-convex pattern 114 may be formed on a surface of acasting roll 110.

In detail, in the concave-convex pattern forming operation, a surface ofa casting roll main body 112 configuring the casting roll 110 may besurface-treated using a shot blasting process, thereby forming theconcave-convex pattern 114 through the shot blasting process.

The shot blasting process is used to treat a surface by projecting ashot ball B to a surface of the casting roll main body 112 of thecasting roll 110. Here, the shot ball B described above may be a shotball used in a shot blasting process according to the related art.Preferably, a shot ball B having a diameter of at least 2.8 mm isprojected at high pressure to form a concave portion of theconcave-convex pattern 114 in a concave form corresponding to a ballshape. In this case, a central part of the concave portion of theconcave-convex pattern 114, pressed by the shot ball B, may be formed tobe concave, and a pretreatment convex portion 114 a, a part connected toanother concave portion of the concave-convex pattern 114 adjacentthereto, may protrude sharply to be connected thereto.

For example, a convex portion of the concave-convex pattern 114 may beconnected to a concave portion of the concave-convex pattern 114adjacent thereto at an angle R of 30 degrees or more.

When the concave-convex pattern 114 is formed in the concave-convexpattern forming operation, the electrolytic polishing operation may beperformed.

In the electrolytic polishing operation, a plurality of convex portionsand concave portions of a concave-convex pattern 114 formed in theconcave-convex pattern 114 forming operation are treated to be rounded.

In detail, in the electrolytic polishing operation, in which the castingroll 110 is immersed in an electrolytic polishing solution to bepolished while being electrochemically dissolved, smoothing of fineconcave and convex portions of 0.01 μm to 0.1 μm and removal ofrelatively large concave and convex portions of tens of μm may beperformed simultaneously, and polishing may be performed simultaneously.

An electrolyte used in the electrolytic polishing operation is mostly anacid solution. In addition, acetic anhydride, alkali, and phosphoricacid are used, and perchloric acid and chromic acid having strongoxidizing power are added thereto.

In the electrolytic polishing operation, the pretreatment convex portion114 a, a protruding part, is relatively quickly dissolved, therebyallowing the convex portion of the concave-convex pattern 114 aftersurface treatment of the casting roll 110 to be smoothly formed.

When the electrolytic polishing operation is performed for a total of 4hours while a current is provided with a voltage of 7 V to 8 V,polishing of the pretreatment convex portion 114 a, a sharply protrudingpart of the concave-convex pattern 114, is quickly performed. Thus, aconvex portion of the concave-convex pattern 114 may be connected to aconcave portion of the concave-convex pattern 114 adjacent thereto at anangle of less than 30 degrees to allow an entirety of the concave-convexpattern 114 to be smoothly formed.

In this case, electrolytic polishing conditions in the electrolyticpolishing operation will be described in Table 1.

TABLE 1 Starting temperature Roll surface Electrolyte Voltage (V)Current (A) Minutes 35.6 35 7.9 1249 120 35.8 45 7.8 1200 60 42 46 7.7986 40 43 51 7.4 250 20

The casting roll 110 manufactured in the surface treatment methoddescribed above may include a surface treated part formed by beingtreated to be rounded overall while the plurality of convex portions andconcave portions of a concave-convex pattern 114 on a surface of thecasting roll main body 112 are electrolytically polished.

As described above, in an exemplary embodiment, while the casting roll110 passes through the electrolytic polishing operation to besurface-treated, a convex portion and a concave portion of theconcave-convex pattern 114 are smoothly connected to each other, andthus, a surface of the casting Y passing through the casting roll 110 tobe manufactured, may have a convex portion and a concave portion of aconcave-convex pattern 114′, which is smooth, corresponding to a surfacetreated part of the casting roll 110. Thus, a foreign substance such asoxide scale or the like is prevented from being inserted into convexportions adjacent thereto.

Even when a portion of a foreign substance is attached to a surface ofthe casting Y, the portion of a foreign substance may be easilyeliminated by rolling oil L or the like in a process of being pressed bya rolling roll 130. Thus, a foreign substance is prevented from beinginserted in a process of being rolled as the thin plate S. The rollingoil L allows frictional resistance between the casting Y and the rollingroll 130 to be reduced, thereby reducing insertion of a foreignsubstance.

Preferably, a convex portion of the concave-convex pattern 114 formed onthe casting roll 110 may be connected at a smooth angle of less than 30degrees when the convex portion thereof is connected to a concaveportion adjacent thereto. In addition, a convex portion of theconcave-convex pattern 114′ formed on the casting Y cast by the castingroll 110 described above may be connected at a smooth angle of less than30 degrees when the convex portion thereof is connected to a concaveportion adjacent thereto.

In an exemplary embodiment, a coating layer formed using a material suchas nickel or the like may be formed on the casting roll 110 to improvewear resistance of a surface of the casting roll main body 112. Inaddition, a surface treated part may be directly formed on the castingroll main body 112, or may be formed on the coating layer.

Example

In the casting roll 110 configured as described above, a convex portionof the concave-convex pattern 114 on a surface treated part is formed tobe rounded. In addition, an angle in which a convex portion of theconcave-convex pattern 114′ of the casting Y manufactured using thecasting roll 110 according to an example is connected to a concaveportion adjacent thereto was measured, and a defect of the thin plate Smanufactured in the form of a finished product by rolling the casting Ythereafter was measured.

With reference to Table. 2, an angle between a convex portion and aconcave portion of the concave-convex pattern 114 formed on a surface ofthe casting Y cast by the casting roll 110 was measured.

When the casting roll 110 according to the related art was used, thenumber, in which an angle between a convex portion and a concave portionof a concave-convex pattern formed on the casting Y is 25 degrees ormore, was 30 per area of 100 mm², and the number, in which an anglebetween a convex portion and a concave portion thereof formed on thecasting Y, is 30 degrees or more, was 10 per area of 100 mm².

A hook defect H may occur in the thin plate S in the form of a finalproduct in which the casting Y is rolled. In this case, a hook defectindex representing the number of hook defects H formed in an area of 1m² was measured as 10.

When the casting roll 110 according to an exemplary embodiment was used,the number in which an angle between a convex portion and a concaveportion of a concave-convex pattern formed on the casting Y is 25degrees or more was 20 per area of 100 mm², and the number in which anangle between a convex portion and a concave portion thereof formed onthe casting Y is 30 degrees or more was 0.

In an exemplary embodiment, a hook defect H does not occur in the thinplate S in the form of a final product in which the casting Y is rolled.

TABLE 2 Angle between convex portion Rolling and concave portion on castmaterial strip surface surface 25 degrees or 30 degrees or hook moremore H index (ea/100 mm²) (ea/100 mm²) (ea/m²) Note Before 30 10 10Related art electrolytic polishing After 20 0 0 Exemplary electrolyticembodiment polishing

As described above, a convex portion and a concave portion of theconcave-convex pattern 114 having an angle of 30 degrees or more on asurface of the casting Y may form a hook defect H in a rolling process.In an exemplary embodiment, a convex portion of the concave-convexpattern 114 is formed on a surface of the casting roll 110 to be roundedby electrolytic polishing to allow roughness to be an angle of less than30 degrees, thereby preventing a hook defect H from being generated in arolling process.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentdisclosure as defined by the appended claims.

1. A surface treatment method comprising: a concave-convex patternforming operation of forming a concave-convex pattern on a surface of acasting roll; and an electrolytic polishing operation ofelectrolytically polishing so that a plurality of convex portions andconcave portions of a concave-convex pattern formed in theconcave-convex pattern forming operation are treated to be rounded. 2.The surface treatment method of claim 1, wherein in the concave-convexpattern forming operation, a surface of the casting roll issurface-treated using a shot blasting process.
 3. The surface treatmentmethod of claim 2, wherein in the shot blasting process of theconcave-convex pattern forming operation, when the casting roll issurface-treated, a shot ball having a diameter of at least 2.8 mm isprojected.
 4. A casting roll comprising: a casting roll main body; and asurface treated part formed on a surface of the casting roll main bodyand formed so that a plurality of convex portions and concave portionsof a concave-convex pattern are treated to be rounded by the surfacetreatment method according to claim
 1. 5. The casting roll of claim 4,wherein convex portions of the concave-convex pattern may be smoothlyconnected to concave portions of the concave-convex pattern adjacentthereto at an angle of less than 30 degrees.
 6. The casting roll ofclaim 4, further comprising a coating layer formed on the surface of thecasting roll main body.